Coiled tubing is used for a wide range of oil well operations, such as drilling, logging, cleanouts, fracturing, cementing, under reaming, fishing, completion, and production operations, among other operations.
A coiled tubing string is typically wrapped around a reel and dispensed onto and off of the reel during an operation. One end of the coiled tubing string extends from the reel to an injector. The injector moves the coiled tubing string into and out of a wellbore. An opposite end of the coiled tubing string is connected, through a coiled tubing swivel, to fluid and/or gas pumps, that pump fluid and/or gas substances through the coiled tubing string during a desired oil well operation. This opposite end of the coiled tubing string rotates along with the coiled tubing reel when the coiled tubing string is dispensed onto and off of the reel.
As such, the coiled tubing swivel is required to provide a high pressure rotating connection between the coiled tubing string and the fluid and/or gas pumps to allow pumped substances to be transferred therebetween. The coiled tubing swivel is typically required to seal: dry gases (such as Nitrogen), liquids (such as water, brine, acid, alcohol, and solvents), foams, and solid suspensions (such as fracturing fluids and jet cleaning materials).
FIG. 1 shows a coiled tubing swivel 10 according to the prior art. As shown, the coiled tubing swivel 10 includes a mandrel 12 for connection to fluid and/or gas pumps, and a hub 14 for connection to a coiled tubing string. As is typical with coiled tubing swivels of the prior art, the depicted swivel 10 includes a right angle turn or an elbow 16. Such coiled tubing swivels wear out very rapidly at the elbow 16 when solids, such as sand, are pumped therethrough.
As shown, the hub 14 includes a first section 18 screw fastened to a second section 20. Before assembly with the hub, a packing 22 and ball bearing assemblies 24 are attached to the outer diameter of the mandrel 12. The ball bearing assemblies 24 each contain an inner ring 26 and an outer ring 28, with spherical bearings 30 disposed therebetween. The ball bearing assemblies 24 are press fit onto the outer diameter of the mandrel 12, and must be precisely aligned both parallel to the mandrel axis and concentric with the mandrel axis. If either ball bearing assembly 24 is misaligned in either of these respects, it will wear and/or fail quickly.
After the packing 22 and ball bearing assemblies 24 have been assembled on the mandrel, the mandrel 12 is inserted into the hub first section 18, and the hub second section 20 is screw fastened to the hub first section 18 to secure the mandrel 12 to the hub 14.
A problem with the above-described coiled tubing swivel 10 is that any replacement of the mandrel 12 requires reinstallation of the ball bearing assemblies 24. Due to the precision required and other difficulties associating with installing the ball bearing assemblies 24 on the mandrel 12 such a replacement of the mandrel 12 becomes a very time consuming process.
Also in order to allow the packing 22 to rotate with respect to the mandrel 12, a coating is applied to the mandrel 12 to create a slick, sliding surface. During use, this coating is placed in tension due to the internal pressure in the mandrel 12 caused by a pumped substance flowing therethrough. This tensioning of the coating can lead to cracks forming therein due to the mismatch in elastic moduli between the mandrel 12 and the coating. These cracks form sites for crevice corrosion, leading to disengagement of the coating from the mandrel 12 and failure in the packings 22. In some prior art swivels this coating has a coefficient of thermal expansion, which is substantially different from the mandrel to which it is applied, and the application of the coating to the mandrel is done at high temperatures. These factors also act to create tension on the coating during use.
FIG. 2 shows an enlarged and slightly exaggerated cross sectional view of a portion of the ball bearing assembly 24 of FIG, 1. As shown, the inner and outer rings 26 and 28 of the ball bearing assembly 24 contain facing races 25 which substantially trace the outer diameter of the spherical bearings 30. However, each race 25 has a side 27 that is thicker, or extends further around the spherical bearings 30, than the other side 29. Also, the ball bearing assembly 24 will not function properly unless the thicker side 27 of the outer ring's race is disposed caddy-corner from the thicker side 27 of the inner ring's race. Thus, creating the possibility of an improper installation of the ball bearing assemblies 24 on the mandrel 12.
Also, this ball bearing assembly 24 configuration creates a reaction force F, which is good at carrying an axial load A, but poor in carrying radial R, thrust T, and bending moment loads B. In order to adequately carry radial R, thrust T, and bending moment loads B, two or more ball bearing assemblies 24 must be used in combination, preferably adjacently positioned on the mandrel 12 (as shown in FIG. 1), and oriented oppositely such that their reaction forces F are oppositely directed. With the reaction forces F oppositely directed, the ball bearing assemblies 24 can react to a larger variety of forces.
Another problem with this ball bearing assembly 24 configuration is that openings 15 on the sides of the assembly 24 expose the spherical bearings 30 to the outside environment, such as dust and debris. Contamination of the bearings 30 by such debris causes the ball bearing assembly 24 to wear and/or fail quickly. Accordingly, a need exists for an improved coiled tubing swivel assembly.